(a) Field of the Invention
The present invention relates to a water-dispersible polyurethane (hereinafter referred to as PU) resin, and a method for preparing aqueous emulsion via water-dispersible polyurethane grafted acrylate copolymerization, and more particularly relates to a copolymerization method for side grafting an acrylic monomer to a water-dispersible PU long chain to enable providing a pollution-free water-dispersible PU copolymer acrylic emulsion, which possesses characteristics of PU and acrylic. The resin emulsion has a large molecular weight, homogeneous distribution, and a good appearance. Its specific gravity is slightly greater than that of water, has excellent storage stability, good permeability, and is easily worked with. The water resistance of a dried coating of the aqueous emulsion is good, and also has high extensibility, excellent tensile strength, good adhesion, and superior wear resistance.
(b) Description of the Prior Art
Traditionally, an aqueous polyurethane emulsion is formed by direct emulsification using a stirring and mixing device with a strong shearing force on a low molecular weight emulsifier from a prepolymer produced from isocyanate and a polyol. However, the stability of the aqueous polyurethane emulsion is poor. If an aqueous polyurethane emulsion of higher stability is required, then a reaction is carried out between an polyisocyanate, a polyol, and a polyol bearing hydrophilic carboxylic monomer to form a polyurethane prepolymer having isocyanate as the terminal group and containing a hydrophilic functional group. Water dispersion is then effected using a high-speed stirring device, after which a diamine compound is added to serve as a chain extender to form the emulsion.
As for the preparation of an acrylic emulsion, an acrylic monomer in an aqueous solution is used, and a low molecular weight emulsifier is adopted to carry out an emulsion polymerization reaction. The monomer in water or other solvent, along with an initiator and an emulsifier, is then mechanically stirred, which together with the effect of the emulsifier causes the monomer to disperse and become an emulsion state; the acrylic emulsion is thus obtained by carrying out free-radical polymerization thereof. However, if production of a solid polymer is required, then the acrylic emulsion must undergo a multi-step procedure involving de-emulsification, washing, dehydration, and drying. This results in high production costs, and the impurities in the polymer emulsion obtained are difficult to remove completely, which will affect the electrical properties of the polymer emulsion. Furthermore, the solid polymer is also inappropriate to work with because of its shortcomings including easily becoming brittle at low temperatures, and being prone to become tacky at high temperatures.
Polyurethane has characteristic properties including excellent low temperature flexibility and wear resistance, and good elasticity. However, for non-polar base materials, polyurethane has shortcomings including poor wettability, intolerance to high temperature, poor water resistance, and poor weathering resistance. Acrylic has good water resistance and weathering resistance, but has the shortcomings of easily becoming brittle at low temperatures, and being prone to become tacky at high temperatures. Hence, there is an urgent need to solve these problems. Accordingly, the task of the present invention is to overcome and resolve each of the aforementioned shortcomings, and make up for their deficiencies by combining the advantages of polyurethane and acrylic.